Transducer and method of making the same



Nov. 1, 1949.

R. B. GRAY 7 2,486,560

TRANSDUCER AND METHOD OF MAKING THE-SAME Filed Sept; 20, 1946 i g R) Q E/Z /Z JOMPR5510N 0 TENS/0A! Fla. 6.

T/ME

IN VEN TOR.

Patented Nov. 1, 1949 TRANSDUCER AND lHETHOD OF MAKING THE SALE RobertB. Gray, Erie Pa assignor to Erie Resistor Corporation, Erie, la.., acorporation of Pennsylvania Application September 20, 1946, Serial No.898,374 15 Claims. (Cl. 171-327) mechanical vibrations to alternatingelectrical voltages of corresponding wave form making use of a ceramicsuch as BaTiO: in tetragonal crystalline state.

In a preferred form the ceramic is subjected to a charging voltage andalso to a mechanical stress while cooling from the temperature range atwhich the crystalline structure transforms from the cubic to thetetragonal. As the ceramic is cooled below the transformationtemperature a portion of the charge is in eflect permanently fixed orbound on the interfaces of the crystals producing in effect a ceramicelectret. Both the charging voltage and the mechanical stress.contribute to the orientation of tetragonal crystals in the preferreddirection. The permanent charge results in a piezo-electric effect whenthe a crystal is subjected to mechanical vibration. The response islinear. One use is for phonograph pickups where the mechanicalruggedness and ability to withstand extreme temperatures are additionalpractical advantages. Other uses are in microphones and speakers.Fin'ther objects appear in the specification and claims.

In the drawing, Fig. 1 is an end elevation of a phonograph pickup; Fig.2 is a perspective view of a modification; Fig. 3 is a perspective ofone of the tetragonal crystals; Fig. 4 is a diagrammatic viewillustrating the production of the piezo-electric effect by the boundcharges; Fig. 5 is a diagram showing the effect of tension andcompression on the dielectric constant; Fig. 6 is a diagram illustratingthe exponential decay of changes in dielectric constant produced bymechanical or electrostatic stress.

Referring to the drawing, l indicates a phonograph needle held in ametal chuck 2 by a thumb screw 3, and 4 indicates a metal block fixed inthe arm. Between the chuck and block is a tubular ceramic condenser 5having its ends fixed in sockets 8 and I in the chuck and block. Anouter metalized coating 8 is soldered to the chuck and an innermetalized coating 9 is connected by a lead in to the block. The coatingsare the condenser electrodes.

The ceramic comprises poly-crystalline BaTiOa having a substantial partin the tetragonal crystalline state. In the manufacture, the BaTiOa ismixed with addition agents such as clay or bentonite primarily for thepurpose of making the mixture easier to work, and is then pressed intoshape and fired at an elevated temperature, e. g.

.2200-2500 F. to form a dense ceramic. High temperature firing. andaddition agents such as to bentonite which promote grain growth andincrease the crystal size are preferred. The metal coatings 8 and 9 arethen applied, for example as a silver paint, and fixed by firing at from250 to 1400 F. The condenser-is then activated by heating the condenserto the transformation temperature of about C. while applying a voltagebetween the coatings 8 and 9 until the condenser has cooled below thetransformation temperature. It is not necessary that the voltage becontinuously applied until the ceramic has cooled below thetransformation temperature, although it is preferable. At temperaturesabove the transformation temperature, or more accurately, above therange of temperatures at which the transformation takes place, BaTiO:has a cubic crystal structure. Below the transformation tempera.- turethe crystal structure is predominantly tetragonal, as shown in Fig. 3,comprising two square end faces of 3.99 Angstrom units on each side andfour rectangular faces having a length of 4.03 Angstrom units. There maybe some cubic crystals remaining. The voltage between the coatingsproduces a permanent polarization probably resulting from charges on theinterfaces of the crystals. In effect the activated condenser is aceramic electret as diagrammatically indicated in Fig. 4.

In the tetragonal state an electric or mechanical stress produces asudden increase in dielectric constant as indicated by the peaks ii inFig. 5. As the stress is continued, the dielectric constant decaysexponentially with time as indicated by the curves I2. The increase indielectric constant is linearly proportional to the stress and twice asrapid for tension stress as for compression stress as indicated by thecurves [3 and H in Fig. 6. Because the slope for compression stress isdifierent from the slope for tension stress, distortion would beintroduced if the change in dielectric constant and the resultant changein capacity between the coatings 8 and 9 were utilized to effect thetranslation between electrical and mechanical vibrations.

when activated to produce the permanent or bound charge, there is alinear translation between alternating mechanical stress along the axisof the condenser and the voltage appearing between the coatings 8 and 9.This is apparently due to elastic deformation of the tetragonalcrystals. It may be due in part to an adiabatic transformation of thetetragonal crystals with the long axis in the direction of stress totetragonal crystals with the long axis at right angles the direction ofstress. Part of the effect may be due to change of the tetragonalcrystals toward the cubic. A vibratory stress in the direction of thearrow II in Fig. 4 produces a movement of the coatings 8 and 9 in thedirection of the arrows l8 and results in the production of acorresponding induced voltage between the coatings 8 and 8. Conversely,the application of an alternating voltage across the coatings 8 and 9results in a corresponding elongation and contraction of the length ofthe condenser. The eflect produced is similar to a piezo-electriceifect. The absence of air between the coatings and ceramic increasesthe piezo-electric eflect. When used as a phonograph pickup, the lateralmovement of the needle as it tracks in the record groove produces anendwise stress on the tubular ceramic and results in a radial movementof the coatings 8 and 8, producing a voltage on the coatings whichappears across leads i1 and i8; the voltage being proportional to theamplitude of the needle movement. The linearity is maintained over theaudio range.

In the pickup shown in Fig. 1, the tetragonal crystals are orientedalong the axis of the ceramic tube by the charging voltage applied tothe coatings 8 and while cooling from the transformation temperature.The orientation is due solely to the electrical stress. In Fig. 2 isshown a pickup in which the ceramic is also mechanically stressed,thereby producing a better or more complete orientation. Both stresseshave the same orienting direction. The added mechanical stress permitsmore complete orientation without excessive charging voltages.

InFig. 2 an angular metal block is fixed in the tone arm has a BaTiO:ceramic disc condenser 29 having a metalized coating 2! soldered on oneface to one arm of the block and a strip spring '22 soldered to theother arm of the block. A metal bracket in the plane of the disc issoldered to the coating 2i and the spring 22 while the spring is benttoward the condenser. The amount of tension is controlled by thestiffness of the spring and the amount of bending. While the ceramic isunder tension, it is heated to and then cooled from the transformationtemperature while a charging voltage is applied betweenthe coating 2iand a; metalcoating 23 on the opposite face. Both the tension and thecharging voltage contribute to orientation of the tetragonal crystals inthe plane of the disc. In use the block, I9, is mounted in the tone armso the spring, 22, extends along the record groove. A needle, 14, in achuck, 25. on the projecting end of the spring transmits the undulationof the groove to the condenser as a mechanical vibration in line withthe orientation of the tetrasonal crystals produced by the springtension. This results in a voltage between the coatings. 2| and 23,appearing in leads, 28 and 21, linearly proportional to the amplitude ofthe needle movement. The voltage is somewhat greater due to the betteror higher percentage preferred orientation of the tetragonal crystals.

Other titanates exhibit the same properties as barium titanate but thetransformation temperature is too low to be practical. There is apossibility of mixing other titanates with barium titanate obtaining asolid solution with a lowered but still practical transformationtemperature. Additions of from 4-5% calcium titanate drop thetransformation temperature to about 55 C. Higher percentages do not dropthe transformation temperature further indicating a saturation of thesolid solution. The excess is in 7s onai crystal line state treated a Iteifect merely an inert ingredient. Additions of strontium titanate gointo solid solution in percentages of over 28% apparently withoutsaturation limit. At 28%, the-transformation temperature is at theimpractically low value of 13 C. A wide variety of ingredients 'whichdonot affect the transformation temperature may be added. Among these areingredients for increasing the workability, for promoting grain growth,for controlling the firing temperature and for other purposes known tothe ceramic condenser art.

Among the advantages of the barium titanate transducer are low cost,mechanical strength and resistance to shock, high output and linearresponse, and stability. The change in performance with temperature andaging is not objectionable. If the activation should be destroyed by anexcessively high temperature ducer can be easily reactivated.

,what 1 claim as new is:

1. A transducer having as an acth/e ingredient; barium titanate intetragonal crystalline state.

2. A transducer having as an active ingredient barium titanate polarizedby bound charges.

3. A transducer having as an active ingredient barium titanate intetragonal crystalline state stressed at the transformation temperatureto orient the crystals in the preferred direction.

4. A transducer having as an active ingredient barium titanate intetragonal crystalline state with the crystals oriented in the directionof vibratory movement.

5. A transducer having as an active ingredient barium titanate intetragonal crystalline state with the crystals oriented in the directionof vibratory movement and polarized in a direction transverse to thedirection of vibratory movement by bound charges.

6. A transducer comprising a ceramic having spaced faces provided withmetalized coatings, said ceramic having as an active ingredient bariumtitanate in tetragonal crystalline state with the crystals orientedalong the faces and polarized in a direction transverse to the faces bybound charges.

7. A transducer comprising a ceramic having spaced faces provided withmetalized coatings, said ceramic having as an active ingredient a solidsolution of barium titanate in tetragonal crystalline state with thecrystals oriented along the faces and polarized in a directiontransverse to the faces by bound charges.

8. A phonograph pickup comprising a ceramic with spaced metalizedcoatings extending along the direction of movement produced by therecord groove, said ceramic having as an active ingredientpoly-crystalline barium titanate in tetragonal crystalline state withthe crystals oriented along said direction of movement and polarizedtransverse to said direction oi movement by bound charges.

9. A phonograph pickup comprising a ceramic the transwith spacedmetalized coatings extending along the direction of movement produced bythe record groove, said ceramic having as an active ingredientpoly-crystalline barium titanate in tetragonal crystalline statepolarized transverse to said direction of movement by bound charges,

10. A phonograph pickup comprising a ceramic with spaced metalizedcoatings extending along the direction of movement produced by therecord groove, said ceramic having as an active ingredientpoly-crystalline barium titanate in tetragto promote grain growth andpolarized transverse to said direction of movement by bound charges.

11. The method of activating a barium titanate ceramic to produce apiezo-electric efiect which comprises subjecting the ceramic to apolarizing voltage at the temperature at which the crystals transformfrom the cubic to the tetragonal.

12. The method of making a transducer which comprises treating a bariumtitanate ceramic to promote grain growth, and subjecting the ceramic toa polarizing voltage at the temperature at which the crystals transformfrom the cubic to the tetragonal.

13. The method of making a transducer which comprises subjecting abarium titanate ceramic to a polarizing voltage and a mechanical stresstending to orient the tetragonal crystals in a. direction transverse tothe direction of polarization at the temperature at which the crystalstransform from the cubic to the tetragonal.

14. The method of making a transducer which comprises subjecting abarium titanate ceramic to a polarizing voltage and a tension stresstransverse to the direction of polarization at the temperature at whichthe crystals transform from the cubic to the tetragonal.

15. The method of making a transducer which comprises subjecting abarium titanate ceramic to a polarizing voltage at the temperature atwhich the crystals transform from the cubic to the tetragonal.

ROBERT B. GRAY.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,995,257 Sawyer Mar. 19, 19352,402,515 Wainer June 18, 1946 2,424,111 Navias July 15, 194'! 2,424,273Haas, Jr July 22, 1947 OTHER REFERENCES W111 and Goldman: Compt. Rend.Acad. Sci. (URSS) 49, 177-180 (1945). Library of Congress SmithsonianDeposit.

Wainer, E.: Electrochemical Society, 89; 331 to 356 (1946).

Cady, W. G.: Piezoelectricity, McGraw-Hill, pp. 4, 198, 233, 234, 235,260, 261-6, 614 (1946).

G. Busch: Helv. Phys. Acta, 11, 269 (1938).

Coursey and others: Nature 156; 480, 717 (1945); ibid. 157; 297-298(1946),

Wul: Compt. Rend. Acad. Sci. (URSS), 46; 139, 154 (1945).

De Bretteville: Jr. Phys. Rev., 69,- 687 (1940).

Donley: R. C. S. Review 8; 539, 553 (1947).

